1. Field of the Invention
The present invention relates to an ink jet printer and method of ink jet printing. More particularly, the present invention relates to an ink jet printer and method of ink jet printing in which occurrence of cockle on a recording medium can be prevented.
2. Description Related to the Prior Art
An ink jet printer is an apparatus known in the field of image forming, and characterized in easily printing an image relatively inexpensively and with a somewhat simple construction. The ink jet printer is widely utilized for printing a photograph, document and the like, and for patterning a printed circuit board, producing a color filter for a display device such as a liquid crystal display panel and the like.
The ink jet printer ejects droplets of ink on to recording medium or recording sheet of paper or the like, and forms an image to produce a printed sheet obtained finally. It is necessary to evaporate the ink sufficiently after ejection to the recording medium before discharging the printed sheet. This is referred to fixation by drying. Should the printed sheet before the fixation by drying be contacted by a finger of a user's hand, a recording head, platen, roller or other mechanical elements in the ink jet printer, or other products of the printed sheet, unwanted transfer of ink occurs to degrade the printed sheet seriously. Furthermore, it is likely to cause smears on succeeding sheets of the recording medium with ink if the ink remains present on the recording head, platen, roller or the like.
Smears of ink before completing the fixation by drying are likely to occur as a considerably serious problem, because time for the fixation by drying cannot be kept during image forming in spite of recent technical development in the printing speed of the ink jet printer. There is a type of ink jet printer capable of double-sided recording in which images are printable on first and second surfaces of the recording medium. If second surface recording is started before completing the fixation by drying of a first surface, smears occur. Various documents disclose suggestion to solve this problem, including JP-A 6-134982, U.S. Pat. No. 7,204,572 (corresponding to JP-A 2005-125750), JP-A 2005-014434 and JP-A 2007-030201.
JP-A 6-134982 discloses an ink jet printer in which a length of waiting time between an end of first surface recording to a start of the second surface recording in the double-sided recording is determined by considering a type of the recording medium. In U.S. Pat. No. 7,204,572 (corresponding to JP-A 2005-125750), a frame on the first surface of the recording medium is split into a plurality of partial areas or unit regions. Information of a specified ink volume of ink, such as a dot number of dots to be recorded, is retrieved and assigned to each of the partial areas. The waiting time is determined according to the retrieved information of the specified ink volume.
In the conventional techniques, the waiting time is predetermined in consideration of severest conditions, such as a combination of art paper and an image developable with the maximum duty of printing. There are situations of very low efficiency with too long waiting time, for example, the use of a type of recording medium with a rapid drying property for ink, an image with a relatively small value of the specified ink volume. However, the suggestions in the documents described above determine the waiting time according to a type of recording medium for use or the ink volume of ink, so that an image can be recorded efficiently.
JP-A 2005-014434 discloses an ink jet printer for printing a postcard in the double-sided recording. A surface of the postcard for filling an address and name is regarded as a specific surface of which the specified ink volume is estimated lower. The first surface is used as the address surface for initially printing the address, name and the like. JP-A 2007-030201 discloses a sequence of comparing the printing duty of images between the first and second surfaces. If the printing duty of the first surface is higher than that of a second surface, the two images to be printed are exchanged with one another between the first and second surfaces. One of the two images with a smaller value of the printing duty or the ink volume can be printed earlier in the first surface recording effectively, as the waiting time required for the fixation by drying can be shorter.
In addition to the problem of smears with ink without completing the fixation by drying, occurrence of cockle or wrinkles in the printed sheet is another serious problem in the ink jet printer.
When the cockles occur, quality of the printed sheet will be lowered due to poor appearance in the printed sheet, high difficulty in the handling and other reasons. The problem of the cockles in the first surface recording in the course of the double-sided recording is specifically serious, because precision of positioning ink droplets will be low in the second surface recording, smears will occur in contact of the recording head and the recording medium, or the recording medium may jam with failure in feeding.
Cockles are created when a moisture content of the recording medium becomes higher than a tolerable amount. To be precise, cellulose fiber constituting the recording medium swells in penetration of moisture of the ink in the recording medium. The cellulose fiber comes to compress upon the fixation by drying. If the extent of the swelling of the cellulose fiber is considerably high, the cockles occur.
In JP-A 6-134982 and U.S. Pat. No. 7,204,572 (corresponding to JP-A 2005-125750), the waiting time is determined according to the type of recording medium or the specified ink volume. However, the moisture in the ink penetrates in the recording medium even in the course of the waiting time. The moisture content in the recording medium is likely to become higher than a tolerable amount. Cockles will occur remarkably in plain paper as a type of recording medium with property of high penetration, because the cellulose fiber swells considerably with the waiting time of an excessive length.
Also, a problem of occurrence of the cockles remains in JP-A 2005-014434 and JP-A 2007-030201 typically when the moisture content in the recording medium is higher than a tolerable amount, even though such documents disclose a method of a selective operation for the first surface recording with one of the surfaces in which the specified ink volume is smaller.